Oxygen probes will be referred to in the following as simply probes for brevity. That line of the two connecting lines of such probes which normally carries a higher potential than the other line is referred to in the following as a high-potential line whereas the other line is referred to as a low-potential line.
Probes and especially those of the Nernst type exhibit from probe to probe a relatively wide distribution in the voltages which in each case are emitted for a specific air ratio, for example, for the exhaust gas of a motor vehicle. This makes checking for the correct connection of such a probe difficult because a voltage for the one probe is still plausible which, for another probe, would already clearly indicate a fault. Typically, the probe voltage of a Nernst probe lies between 850 mV for measuring a rich mixture and at approximately 100 mV for measuring a lean mixture. However, a first probe can indicate, for example, almost 1 V when measuring a rich mixture and another can measure approximately -80 mV when measuring a lean mixture. 0 V are measured if a short circuit is present between the two wires of a two-wire pair. For the second-mentioned probe, this is a plausible measurement value since even negative voltages can occur which are, however, not evaluated by the typical evaluation circuits; instead, the negative voltages are made equal with a voltage of 0 V. Plausible voltages also occur, for example, when a probe line has become separated from a control apparatus.
In order to reliably determine two-wire short circuits or line separations, the conventional procedure has been that, when the probe voltage has exhibited the value 0 V or the voltage of an ancillary voltage source over a pregiven time span, then the air/fuel mixture is arbitrarily slightly enriched. If the probe signal does not respond to this enrichment, then this is a reliable indication that a fault is present. It is a disadvantage of this test method that the mixture must be enriched which leads to an increased exhaust-gas discharge and also causes other disadvantages.
Oxygen probes are mostly so arranged that they determine the gas composition in the exhaust-gas flow forward of a catalytic converter. However, it is also known from U.S. Pat. Nos. 3,962,866 and 4,622,809 to provide an additional probe rearward of a catalytic converter in order to monitor the converting capacity of the catalytic converter with the aid of this probe. As long as the catalytic converter provides excellent conversion, a gas mixture of a very uniform composition flows past the rearward probe during steady-state operation of the corresponding internal combustion engine. Then, a voltage which is essentially constant is continuously measured. This fact leads to difficulties in checking the operability of the probe rearward of the catalytic converter since, for a probe voltage which remains continuously constant, it is unclear whether the probe is no longer correctly connected or whether the conversion of the catalytic converter is so good that no changes occur. A check is however possible even in this case in that a mixture is arbitrarily generated over a longer time which deviates from the air ratio one. This leads to the condition that a rich or lean mixture also occurs at the output of the catalytic converter. This causes the above-mentioned disadvantages.